A gas boiler for producing hot water normally comprises a gas burner, and at least one heat exchanger through which combustion fumes and water flow. Some types of gas boilers, known as condensation boilers, condense the steam in the combustion fumes and transfer the latent heat in the fumes to the water. Condensation boilers are further divided into a first type, equipped with a first exchanger close to the burner, and a second exchanger for simply condensing the fumes; and a second type, equipped with only one heat exchanger which provides solely for thermal exchange along a first portion, and for both thermal exchange and fume condensation along a second portion.
International patent application WO 2004/090434 discloses a condensation or dual-function exchanger of the above type, which comprises a casing extending along a first axis and through which combustion fumes flow; a tube along which water flows, and which is housed inside said casing, and coils about the first axis to form a helix comprising a succession of turns; and deflecting means for directing the fumes between successive turns of a first helix portion in a first direction and between successive turns of a second helix portion in a second direction opposite to the first direction;
Tubes are often finned and/or provided with walls having asymmetric cross-sections so as to increase the heat exchange between water and fumes. However, in a heat exchanger of the above type, even though fins and/or particularly shaped walls may in crease the heat exchange when fumes flow in a first direction between adjacent turns, the same fins and/or particularly shaped walls may disfavour heat exchange when fumes flow in a second direction opposite to first direction.